The present disclosure relates a high voltage direct current (HVDC) transmission system, and more particularly, to an HVDC system that may compensate for the deviation of voltage values measured through a plurality of potential transformers.
High voltage direct current (HVDC) transmission indicates converting alternating current (AC) power produced at a power station into DC power by a transmission site to transmit the DC power, and then re-converting the DC power into the AC power by a reception site to supplying the AC power.
An HVDC transmission system is applied to submarine cable power transmission, large-amount long-distance power transmission, interconnection between AC systems, etc. Also, the HVDC transmission system enables different frequency systems interconnection and asynchronism interconnection.
The transmission site converts the AC power into the DC power. That is, since transmitting the AC power by using a submarine cable is significantly dangerous, the transmission site converts the AC power into the DC power to transmit the DC power to the reception site.
There are many types of voltage source inverters used in the HVDC transmission system, and a modular multi-level voltage source converter is recently receiving attention most remarkably.
A modular multi-level converter (MMC) is a device converting DC power into AC power by using a plurality of sub modules and operates by allowing each of the sub modules to have a charging, discharging and bypass state.
The potential transformers are installed at many locations in the HVDC transmission system for the purpose of system control and system protection.
However, the potential transformers may have an error of measurement because they measure different measurement values according to a voltage measurement range even when they are the same product, which is recognized as a system fault and thus there may be a need to stop system operation in a serious case.
That is, the voltage measurement error of a single potential transformer generally needs to have a range of about 0.2% to about 0.5%. However, a potential transformer applied to the HVDC transmission system experiences a significant increase in an error range when measuring a high voltage, and when a plurality of potential transformers are installed at many locations, the error range caused by the error of measurement that each potential transformer has, has no choice but to increase.
To this end, it is also possible to use a more precise sensing device, but there are limitations in installation cost and technology in order to use the more precise sensing device.